MANY SNP supporters were “driven away” by the party’s strong anti-Brexit stance, leading to “heavy losses” at the General Election, the definitive study of the vote has confirmed.

Academics from the British Election Study (BES) found 4 in 10 voters who backed the SNP in 2015 and then voted Leave switched to another party because of Europe.

Professor Ed Fieldhouse and Chris Prosser of Manchester University also found the Scottish Tories prospered by picking up both Leave and Remain voters opposed to independence.

Despite the UK Tories being regarded as pro-hard Brexit, Ruth Davidson’s party benefited from a “softer approach” to leaving the EU and a strong Unionist stance, they said.

The SNP suffered its worst election reverse since 1979 in June, losing 21 of its 56 seats as its vote share fell from 50 to 36.9 per cent and its vote tally from 1.45m to 978,000.

The BES researchers were able to track the choices of the same people across the 2014 independence referendum, the 2016 EU referendum and the 2015 and 2017 elections.

Based on the two referendums, around a third of their sample were No/Remain voters, a quarter Yes/Remain voters, a fifth No/Leave voters and around 17 per cent Yes/Leave.

“The EU referendum clearly cut across the divisions over Scottish independence,” they said.

Despite Yes voters from 2014 sticking overwhelmingly with the SNP in 2015, there was a marked shift following the Brexit vote.

“Yes/Leave voters were much more likely to defect from the SNP in 2017, with 4 in 10 switching to another party, with similar proportions going to the Conservatives and Labour.

“It appears that, just as Labour’s position on the Independence referendum lost them votes to the SNP, many 2015 SNP voters were driven away by the party’s strong pro-Remain stance,” they wrote.

The Unionist vote also split, with Labour losing a fifth of its No/Remain backers to the Tories, and a smaller fraction to the LibDems.

“Although nationally the Conservatives were more clearly associated with a hard Brexit, the softer approach from the Scottish party, together with a strong position against a second independence referendum and Ruth Davidson’s effective leadership, clearly resonated with this group,” the BES team said.

The Scottish Tories doubled their support among No/Leave voters to two-thirds of the group after picking up half the 2015 Labour voters, 6 in 10 LibDems and most Ukippers.

Professor Fieldhouse and Mr Prosser said the Scottish party system had been “completely transformed” since the 2010 General Election, as the SNP rose from third to first in MPs, Labour sank from first to third, and the Tories went from fourth to second.

They said: “It is not hard to see how the referendums on Scottish independence and the UK’s membership of the EU have been the catalyst for these changes.

“In 2015 the Yes side rapidly shifted to the SNP. In 2017, Yes/Remain voters stayed loyal to the SNP but they suffered heavy losses amongst Yes/Leave voters.

“The relative success of the Conservative in attracting a substantial minority of No/Remain voters in spite of the national party’s hard line position on Brexit made a significant contribution to their increase in vote share. In contrast, Labour performed relatively poorly amongst the No/leave group, losing a large proportion of voters to the Conservatives.

“The ability of the Conservatives to capture No/Remain voters as well as No/Leave voters is may be partly attributable to their strong campaign, but also reflects the relative importance of the two referendums in defining Scottish voters political identities.”

Looking at the election across the UK, the pair found voters regarded Labour as the “best bet” for maintaining close ties with the EU despite its “ambiguous” single market position.

Drawing on more a sample of more than 30,000 voters over successive elections and referendums, they concluded Jeremy Corbyn's party won over “large numbers” of Remain backers from the Tories, Greens and even the stridently pro-EU LibDems.

More than half of Remain voters in 2016 went on to support Labour in the snap election, compared to a quarter voting Tory and 15 per cent siding with the LibDems.

Prof Fieldhouse and Mr Prosser added of the election data: "One of the reasons Labour could do so well amongst Remainers is that by May 2017 the Brexit debate was not so much about whether or not to leave or remain, but about how to leave the EU."

The BES assessment of who Leave and Remain backers supported at the election was based on a survey by YouGov between June 9 and June 23, involving 31,196 respondents.

Prof Fieldhouse and Mr Prosser said respondents were asked if it was more important for the UK to protect EU single market access or gain full control of immigration in Brexit talks.

They found a “striking correlation” between wanting to control immigration and voting Tory on one hand, and wanting single market access and voting Labour or LibDem on the other.

"The Conservatives lead Labour by more than 40 percentage points amongst those most in support of full control of immigration, with Labour having a similar lead among those wanting complete access to the single market. So to put it another way, the Tories were the party of hard Brexit whilst Labour was the party of soft Brexit.”

They suggested that, despite pushing a second EU referendum, the LibDems failed to prosper because of the “lingering effects of coalition government, ineffective leadership and a realisation amongst Remain voters that LibDems could not win in most seats”.

Meanwhile the hard-line Tory position on Brexit and absence of Nigel Farage led to a collapse of the Ukip vote, with more than half of 2015 Ukip voters switching to the Tories.